This invention relates to radio communication receivers in general and particularly to receivers that decode digital signals transmitted over radio links to remote locations.
Such receivers can comprise selective call receivers of the type commonly utilized in radio paging systems. A selective call receiver is a receiver that responds and alerts the user to calls that are directed to it only and not generally to all calls on a frequency or channel. Conventionally, such radios recognize messages being transmitted to it by the particular address information of the transmitted signals. Commonly used address information signals include sequential tone signals comprised of multiple tones, and digitally encoded binary frequency shift keying (FSK) signals.
Digital code receivers include decoders that operate at approximately the bit rate of the transmitted digital signal, and serve to compare the signal patterns received from the transmitter with the signal patterns assigned to the pager. All digitally encoded pagers in use today are designed to operate at particular predetermined bit rates. For example, a receiver for a POCSAG system is designed to operate at 512 bits per second (bps), while a receiver for a Golay Sequential code (GSC) system is designed to decode addresses at 300 bps. A display pager in a Golay system decodes data messages at 600 bps.
As a general principle, the lower the bit rate of transmission the higher the sensitivity of the paging receiver and thus the higher the reliability regarding reception and decoding of the signal. Conversely, as the transmitted signal bit rate is increased the sensitivity of the receiver and reliability and accuracy of reception and decoding decreases. This is particularly pronounced in fringe areas where the radio signals may be subject to interference or fading.
In present selective call or radio paging systems, the system transmission bit rate is preselected to provide an acceptable level of accuracy for reception of digitally coded signals. Once this bit rate is chosen, the maximum of number of radio addresses of a given length that can be transmitted during any time interval is therefore determined.
System loading over a 24 hour period of a fully loaded paging system such as may be found in major metropolitan areas is illustrated in FIG. 6. In this illustration, the system is operating at 100 percent of full capacity during the day from 10:00 A.M. until 4:00 P.M. This means, that the transmitter is transmitting at 100 percent of its through-put capacity during this 6 hour interval during which the channel is fully loaded. New messages placed in the system during this time interval can encounter delays of up to 15 minutes as they are queued, waiting their turn for transmission. If the effective bit rate of transmitted signals in the system could be increased during this time interval, then the message through-put during these peak times could be increased thereby reducing or eliminating such queing delays. Conversely, during lull periods if the bit rate of transmitted signals in the system could be decreased, greater accuracy could be achieved without affecting system through-put.